The present invention relates to vacuum filtration apparatuses with horizontal filter-like conveyors for the filtration of various type of slurries (insoluble solid particles suspended in a liquid) used in, e.g., inorganic chemical processes, organic chemical processes, food processing, mining extraction, or waste treatment processes. In particular, the present invention relates to a vacuum filtrating apparatus for filtering slurries of water and relatively large particles between 20 and 1000 .mu.m in diameter.
Conventionally, vacuum filtrating apparatuses such as those mentioned above are categorized into one of two types. One type continuously draws water from a slurry by means of one or more vacuum chambers. The other type intermittently draws water from a slurry by means of one or more vacuum chambers having one or more valves and the like.
The former is further categorized into one of two types. In one type, vacuum chambers are provided under an endless rubber belt. The endless rubber belt is synchronously advanced with an endless filter belt, wherein continuous filtration can be accomplished. In the other type, a large number of vacuum chambers are coupled with a circulating structure like an endless filter belt, whereby continuous filtration can be accomplished. In both these prior arts, by virtue of the continuous filtration, it is advantageous that the slurry be fed constantly and uniformly. However, the constructions and arrangements of the machinery to accomplish this are inevitably complicated, thereby increasing the costs of the apparatus as a whole.
Examples of the latter, that is, the intermittently suction type, are disclosed in GB-A-1416186 and GB-A-1434211. GB-A-1416186 discloses an endless filter belt which circulates continuously in one direction, and a subjacent suction box provided within the path of the endless filter belt. The suction box is temporarily advanced along with the endless filter belt in such a manner that the suction box is disposed under the horizontally advancing section of the endless filter belt. During the advancement of the suction box, the suction box draws water from the slurry which is conveyed by the endless filter belt. Once the suction box arrives at a predetermined point, the suction box is released from the endless filter belt by the introduction of air by means of the switching of a valve, and the suction box is then quickly returned to the initial point. In this art, by virtue of the continuous advancement of the endless filter belt, it is also advantageous that slurry be fed constantly and uniformly. However, in order to alter the internal pressure of the suction box from sub-atmospheric pressure to atmospheric pressure, and in order to quickly return the suction box to the initial point, the constructions and arrangements of the machinery are again inevitably complicated, thereby increasing the costs of the apparatus as a whole.
GB-A-1434211 discloses an endless filter belt which circulates intermittently in one direction, and a plurality of vacuum trays fixedly secured under the horizontally advancing section of the endless filter belt in such a manner that the endless filter belt can advance over the vacuum trays. The endless filter belt advances and stops intermittently. When the endless filter belt stops, the vacuum trays turn to be evacuated so as to draw water from the slurry conveyed by the endless filter belt. After a certain duration, the interior pressure of the vacuum trays is converted to atmospheric pressure, thereby stopping the suction. This process is repeated so that the slurry is progressively filtered.
In the art of GB-A-1434211, the constructions and arrangements of the machinery is simple, and the cost of the apparatus is therefore less in comparison with the preceding prior arts. However, since the endless filter belt advances intermittently and slurry is fed from a fixed position, the slurry is not uniformly distributed on the endless filter belt. Accordingly, the filtration of the slurry is not uniform. Additionally, the cake of particles formed on the endless filter belt as a result of the filtration of the slurry is formed variably on the endless filter belt so that the washing of the cake on the endless filter belt cannot be performed efficiently. In addition, since the endless filter belt advances intermittently, the switching of the valve mechanisms and the like for the vacuum trays, for the slurry supply, and for the washing water, are elaborate, and the service life of the valve mechanisms and the like is shortened. Furthermore, in order to prevent the reverse movement of the endless filter belt during intermittent advance, a special mechanism is required. Moreover, when washing the cake on the endless filter belt, if the washing water is supplied to the endless filter belt continuously, some portion of the water must be wasted since the endless filter belt presents cakes to the washing water only intermittently.
In order to solve the above problems, a vacuum filtrating apparatus and a method therefor, disclosed in JP-A-62-114616, were proposed by one of the present inventors. In this art, an endless filter belt intermittently advances and a slurry is fed from a feeder, which can reciprocate, onto the endless filter belt, so that a plurality of layers of caked material of uniform thickness is formed on the endless filter belt.
However, in this structure, the caked material generally does not form a uniform thickness on the endless filter belt because the slurry is fed from a feeder which temporally stops. In practice, the slurry does not usually disperse on the endless filter belt uniformly. In particular, if the filtration rate of the slurry is high, the cake of particles may be thicker in some regions. Therefore, it is difficult to filter water from the slurry uniformly.
Since the cakes thus formed are not of uniform thickness, it is difficult to uniformly wash the particles therein.
Water for washing the cake on the endless filter belt is sprayed as a mist by means of a spray nozzle. In order to avoid the clogging of the spray nozzle, the water must be fresh; secondary water which has already been used for washing the cake and then been filtrated sometimes includes fine solid particles and is unacceptable.